I do abs atleast everyother day. I do them usually till I'm exahusted, it's not a compleate waste of time either, it's better than doing nothing think of it that way. I know on the ab machine we have here at school just by doing some ab excercises with a partner like where you hold their ankles and they push your legs different directions until you can't do anymore, then I'll get on this thing like what you would do tri-cep dips on and i just raise my legs straight out like im sitting in mid air until i cant do anymore and then ill do a set of crunches until exhaustion, it takes about 10-15mins to do all 3 of them with a partner and everything and i went from being able to do 160 on the ab machine to 240 in about 9 weeks. So strength has really increased. And I already have a 6-pack really so its the matter to me of getting them stronger. I feel comfortable with doing what I'm doing and i dont see anything other than benefits from it. Just my logic to the situation, but just find what works for you and what you are aiming to do and go for it, goodluck man.